Ultra-broadband optical devices that can operate at wavelengths ranging from the ultra-violet (UV) to the long-wave infra-red (LWIR) have numerous applications, for example, in spectroscopy, in countermeasures for defense technology, and as sensors for gas tracing. Such devices need to be able to generate, modulate, process (filter, split, and/or combine) and/or detect optical signals at a wide range of wavelengths. Currently, such systems are typically composed of discrete fiber-optic or bulk optic elements, with each separate element operating at a different wavelength range. It would be advantageous to integrate multiple devices on a single optical chip that can handle a wide range of optical signals.
A brief discussion of the existing technology is set forth below.
Silicon, silicon oxide, and silicon nitride are the most widely used materials for passive photonic integrated circuits. However, the bandwidth of waveguides made in these materials is limited, and no single combination of waveguide materials spans the full desired range of UV to LWIR. See Richard A. Soref, Stephen J. Emelett, and Walter R. Buchwald, “Silicon Waveguided Components for the Long-wave Infrared Region,” J. Opt. A: Pure Appl. Opt. 8, 840 (2006); and Richard Soref, “Mid-infrared photonics in silicon and germanium,” Nature Photonics 4, no. 8 (2010): 495-497.
More specifically, silicon nitride-on-insulator (NOI) waveguides with silicon oxide cladding have been employed to cover the 0.3 μm to 2.0 μm wavelength range, while silicon-on-insulator (SOI) technology has been used to cover the 1.1 μm to 3.5 μm range. However, the silicon oxide cladding limits the SOI transmission at longer wavelengths, and so in some cases it can be replaced by other materials, e.g., diamond, sapphire or silicon nitride, to extend the range of transmitted wavelengths. See Di Liang, Marco Fiorentino, Shane T. Todd, Geza Kurczveil, Raymond G. Beausoleil, and John E. Bowers, “Fabrication of Silicon-on-Diamond Substrate and Low-Loss Optical Waveguides,” IEEE Phot. Tech. Lett. 23, 657 (2011); Tom Baehr-Jones, Alexander Spott, Rob Ilic, Andrew Spott, Boyan Penkov, William Asher, and Michael Hochberg, “Silicon-on-sapphire integrated waveguides for the mid-infrared,” Optics Express 18, no. 12 (2010): 12127-12135; and Saeed Khan, Jeff Chiles, Jichi Ma, and Sasan Fathpour, “Silicon-on-nitride waveguides for mid- and near-infrared integrated photonics,” Applied Physics Letters 102, no. 12 (2013): 121104.
SOI and silicon nitride waveguides can be combined on a single chip, as shown in Jared F. Bauters, Michael L. Davenport, Martijn J. R. Heck, John Gleason, Arnold Chen, Alexander W. Fang, and John E. Bowers, “Integration of Ultra-Low-Loss Silica Waveguides with Silicon Photonics,” Proc. IEEE Photonics Conf. (Burlingame Calif., 2012). This work also reported mode converters to couple the light from the silicon waveguide to the silicon nitride waveguide. However, since silicon oxide is the cladding for both waveguides, the transmission window is limited to the silicon oxide bandwidth (taking into account that only the exponential tail of the waveguide mode overlaps with the silicon oxide).
Compound semiconductor laser diodes can be suitable optical sources because they can emit over a wide wavelength range, depending on material composition and epitaxial design. Typically, gallium nitride lasers operate in the blue to UV range, gallium arsenide in the red to near-infrared (NIR) range, indium phosphide in the 1.2-2 μm range, InGaAsSb in the 2-3 μm range (see Gela Kipshidze, Takashi Hosoda, Wendy L. Sarney, Leon Shterengas, and Gregory Belenky, “High-Power 2.2-μm Diode Lasers With Metamorphic Arsenic-Free Heterostructures,” IEEE Phot. Tech. Lett. 23, 317 (2011)); interband cascade lasers (ICLs) in the 3-4 μm range (see Igor Vurgaftman, Chadwick L. Canedy, Chul Soo Kim, Mijin Kim, William W. Bewley, James R. Lindle, Joshua Abell, and Jerry R. Meyer, “Mid-Infrared Interband Cascade Lasers Operating at Ambient Temperatures,” New J. Phys. 11, 125015 (2009)); and quantum cascade lasers (QCLs) in the 4-10 μm range (see Y. Bai, N. Bandyopadhyay, S. Tsao, S. Slivken, and M. Razeghi, “Room temperature quantum cascade lasers with 27% wall plug efficiency,” Appl. Phys. Lett. 98, 118102 (2011)). Although these classes of semiconductor laser are in some ways conceptually similar, the conventional methods used to fabricate lasers emitting in diverse spectral regions employ a wide range of materials, substrates, and epitaxial layer designs, which has prevented the monolithic integration of these lasers onto a single chip.
Heterogeneous integration, i.e., combining different materials on a single chip by means of wafer or die bonding, can in principle allow different materials to be combined in a photonic integrated circuit. For example, a laser epitaxial wafer or die can be bonded to a processed silicon chip, see Alexander W. Fang, Hyundai Park, Oded Cohen, Richard Jones, Mario J. Paniccia, and John E. Bowers, “Electrically Pumped Hybrid AlGaInAs-Silicon Evanescent Laser,” Opt. Expr. 14, 9203 (2006); or silicon nitride circuit, see WO 2014/047443 A1, “Integrated dielectric waveguide and semiconductor layer and method therefor.” In this way, sources can be added to the passive silicon or silicon nitride photonic integrated circuits. Different-bandgap materials can be integrated by using selective area bonding. See Hsu-Hao Chang, Ying-hao Kuo, Richard Jones, Assia Barkai, and John E. Bowers. “Integrated hybrid silicon triplexer,” Optics Express 18, no. 23 (2010): 23891-23899.